Photometric circuit having a plurality of photoelectric transducer elements capable of being selectively enabled for photometry

Abstract

A photometric circuit comprises an operational amplifier including a plurality of differential amplifiers, a constant current bias circuit for supplying a constant current bias to the differential amplifiers, and a bias control switching circuit which selectively connects the constant current bias circuit to one of the plurality of differential amplifiers in response to an external signal. A plurality of photoelectric transducer elements, used for purpose of photometry, are connected to differential inputs corresponding to the plurality of differential amplifiers, and the transducer element is selectively enabled for photometry in accordance with the external signal.

What is claimed is: 
     
       1. A photometic circuit having a plurality of photoelectric transducer elements each capable of being selectively enabled for photometry, comprising: a first operational amplifier including a plurality of differential amplifiers, each differential amplifier having a pair of input terminals and an output terminal, the pairs of input terminals of each of said differential amplifiers respectively defining a set of inverting and non-inverting input terminals of the operational amplifier and the output terminals of said differential amplifiers being coupled in common and defining the output of the operational amplifier;   a constant current bias circit for selectively supplying one of said differential amplifiers with a constant bias current, and a bias control switching circuit which selectively connects the constant bias current circuit to the differential amplifier selected by the bias control switching circuit in response to an external control signal applied at a control input of the operational amplifier; and   each of said photoelectric transducer elements being respectively connected across a pair of input terminals of each of said differential amplifiers.   
     
     
       2. A photometric circuit according to claim 1 in which the operational amplifier includes a plurality of logarithmic compression transistors each having collector, base and emitter terminals, each emitter terminal being connected to the output of the operational amplifier, each collector terminal being connected to an associated inverting input terminal and each base terminal being connected to an associated non-inverting input terminal, for generating a logarithmically compressed photometric output at the output of the operational amplifier. 
     
     
       3. A photometric circuit according to claim 1 wherein each differential amplifier has first and second output terminals; a current mirror circuit coupled to each output terminal for generating a mirror current equal to the current in the output terminal coupled thereto;   means responsive to the difference between the mirror currents of the operative differential amplifier for developing a voltage representative of a photometric output.   
     
     
       4. A photometric circuit according to claim 1 wherein the current bias circuit comprises a plurality of current mirror circuits each coupled to one of the differential amplifiers for providing its associated differential amplifier with a predetermined bias current; said bias control switching circuit activating the current mirror circuit of the differential amplifier selected by said external control signal.   
     
     
       5. A photometric circuit according to claim 1 in which a plurality of logarithmic compression diodes are respectively connected between each one of the inverting input terminals and the output terminal of the operational amplifier, whereby a logarithmically compressed photometric output is derived at the output of the operational amplifier. 
     
     
       6. A photometric circuit according to claim 1 in which the input terminals of the differential amplifiers defining non-inverting input terminals of the operational amplifier are coupled in common to define a common non-inverting input terminal. 
     
     
       7. A photometric circuit according to claim 1 further comprising current mirror circuits respectively coupled between the outputs of the differential amplifiers and the output of the operational amplifier and being responsive to the operative differential amplifier for generating mirror currents equal in magnitude to the currents generated by the operative differential amplifier; and means responsive to the difference between said mirror currents for developing a voltage representative of a photometric output at the output of the operational amplifier.   
     
     
       8. A photometric circuit according to claim 7 further comprising logarithmic compression means coupled between the output of said operational amplifier and said photoelectric transducer elements and responsive to said photometric output for creating an effective short circuit condition across the operative photoelectric transducer element. 
     
     
       9. A photometric circuit according to claim 1 in which the input terminals of the differential amplifiers defining inverting input terminals of the operational amplifier are coupled in common to define a common inverting input terminal. 
     
     
       10. A photometric circuit according to claim 9 comprising a second operational amplifier, having inverting and non-inverting input terminals and an output, in which the first operational amplifier is connected in a voltage follower configuration and has its output connected to the inverting input terminal of said second operational amplifier, the second operational amplifier having its output connected to each of the non-inverting input terminals of the first operational amplifier through a logarithmic compression diode, thereby allowing a logarithmically compressed photometric output to be derived at the output terminal of said second operational amplifier.